The present invention relates to a trigger assembly for an electric power tool, and to a switch module particularly but not exclusively for use in the trigger assembly.
The operation of an electric hand drill is typically controlled by means of a pull-trigger, which is used to switch on and off the motor as well as to adjust its speed/torque during operation. As the switch for switching the motor needs to handle a large inductive current, its construction is under stringent requirements in terms of switching time and contact pressure, for example, on one hand and simplicity and compactness on the other hand.
In a conventional construction of the switch, a flat-V-shaped moving contact is pivoted about its apex and acted upon on its inner surface by a spring-loaded plunger. The plunger is laterally slidable along the inner contact surface across the apex, thereby rocking the contact into contact with or out of contact from a fixed contact. Such a switch construction is found to be unsatisfactory in terms of the aforesaid requirements, as the spring action shifts relative to the moving contact. Also, the switch is cumbersome to install, especially within the confined space in the body of the pull-trigger.
The invention seeks to mitigate or at least alleviate such a problem by providing a trigger assembly for an electric power tool.
According to a first aspect of the invention, there is provided a trigger assembly for controlling the operation of an electric power tool including an electric motor. The assembly has a base for fixing within the power tool and a trigger supported by the base for inward and outward movement relative to the power tool. A mechanical switch in the base is operable to switch on and off the motor. An electronic operating circuitry including a solid-state device in the base controls the operation of the motor when the switch is closed. A slider is guided within the base for movement by or with the trigger to operate the switch and the operating circuitry. The switch has a fixed contact, a moving contact pivoted for movement between a first position in contact with the fixed contact and a second position out of contact therefrom, and resilient means biassing the moving contact towards the first position. The moving contact has a first end for contacting the fixed contact and a second end for engagement by the slider to cause pivoting of the moving contact to the second position against the action of the resilient means.
Preferably, the trigger is resiliently biassed towards an outermost home position, and the switch is open when the trigger is in the home position and will be closed when the trigger is moved away from the home position.
More preferably, the switch is closed immediately after the trigger has moved away from the home position and will remain closed thereafter until the trigger returns substantially to the home position.
In a specific construction, the moving contact is pivoted by a stationary conductive support, and the resilient means co-acts between the moving contact and the support.
More specifically, the moving contact is supported adjacent its second end by the support, and the resilient means acts upon the moving contact at about its mid-length.
It is preferred that the slider has a part extending substantially parallel to the direction of movement of the slider for slidably engaging the second end of the moving contact to close the switch.
It is further preferred that the slider part has a surface for slidably engaging the second end of the moving contact, and the surface is discontinuous to minimise the risk of breakdown or flashover therealong.
It is yet further preferred that the slider part surface includes a groove.
In a preferred embodiment, the trigger assembly includes a second mechanical switch in the base connected in parallel with the solid-state device and operable by the slider at or adjacent an innermost position of the trigger to close and thus bypass the solid-state device. The second switch has a fixed contact, a moving contact pivoted for movement between a first position in contact with the fixed contact and a second position out of contact therefrom, and resilient means biassing the moving contact towards the first position. The moving contact has a first end for contacting the fixed contact and a second end for engagement by the slider to cause pivoting of the moving contact to the second position against the action of the resilient means.
Preferably, the trigger is resiliently biassed towards an outermost home position, and the first switch is open when the trigger is in the home position and will be closed when the trigger is moved away from the home position.
It is preferred that both moving contacts are pivoted by a common stationary conductive support, and each resilient means co-acts between the corresponding moving contact and the support.
Preferably, the slider has a first part extending substantially parallel to the direction of movement of the slider for slidably engaging the second end of the moving contact of the first switch to close the first switch. The slider includes a second part on one side of the first part for slidably engaging the second end of the moving contact of the second switch to close the second switch. The second part is shorter than the first part in the direction of outward movement of the trigger.
More preferably, the slider parts have a surface for slidably engaging the second ends of the moving contacts, and the surface is discontinuous to minimise the risk of breakdown or flashover therealong.
Further more preferably, the slider part surface includes a groove.
It is preferred that the first and second switches have substantially the same construction and are operable by the slider in substantially the same manner but at different times according to the moving position of the trigger.
The invention also provides an electric power tool including an electric motor and the aforesaid trigger assembly.
In one example, the electric power tool is an electric hand drill.
According to a second aspect of the invention, there is provided a switch module for use in a controller for controlling the operation of an electric power tool. The switch module comprises two contact levers, each having a first part and a second part along its length, and a common conductive support having an upper portion engaging and supporting the levers by their first parts for individual limited pivotal movement between upper and lower positions and including a lower portion. A spring co-acts between the second part of each lever and the lower portion of the support for resiliently biassing the lever towards the lower position. The spring acts in a direction that reinforces the engagement between the first lever part and the upper support portion such that both levers and the support together form a unitary module. One of the lever acts as a moving contact of a main switch for switching on and off said power tool, and the other lever acts as a moving contact of a bypass switch for continuously switching on said power tool.
Preferably, the upper portion of the support has two upwardly facing recesses, each engaging therein the first part of a corresponding lever.
More preferably, each recess has a restricted opening narrower than the first part of the corresponding lever for retaining it therein.
Further more preferably, each recess has a laterally inward protrusion restricting the recess opening.
It is preferred that each recess has two opposite sides, and the first part of the corresponding lever has a pair of notches on opposite sides thereof inter-engaging with the respective sides of the recess.
Preferably, each spring comprises an extension coil spring having opposite ends connected to the second part of the corresponding lever and the lower portion of the support respectively.
It is preferred that the lower portion of the support includes an integral extension for electrical connection.